This invention relates to a method for detecting a leak before rupture, and more particularly, relates to a method for detecting a leak before rupture using a tearing instability approach in pipeline applications.
Cracks or crack fields can initiate and grow in a pipeline by stress corrosion cracking, fatigue, or corrosion fatigue. The pipeline leaks if one of the cracks penetrates through the wall. The prompt detection of the leak can serve as an early warning and remedial actions can be taken to avoid a subsequent catastrophic failure. It is practically important, therefore, to develop analysis methods that predict conditions for a “leak-before-rupture”.
The first incident of external stress corrosion cracking (SCC) on natural gas pipelines occurred in the mid 1960's and hundreds of failures have occurred since that time. Stress corrosion cracking (SCC) is the formation of brittle cracks in a normally sound material through the simultaneous action of a tensile stress and a corrosive environment.
Evaluation of leak-before-rupture for crack containing pipelines is an area that needs to be addressed for integrity assessment. Currently, fracture mechanics based industry standards, such as API 579-2000, provide detailed procedures for leak-before-rupture assessment using a Level I or III Failure Assessment Diagram (FAD) analysis. Since these procedures only predict crack initiation rather than a catastrophic failure for materials that exhibit stable crack growth by ductile tearing, the results are inconsistent with leak incidents observed in the field.
The criteria for a leak before rupture require that (1) the largest initial crack size left in the structure (e.g., pipeline wall) will not lead to fracture during the life of the component and (2) the largest length of a through-wall crack is less than that which catastrophic rupture will occur for all applicable load cases using either a Level II or Level III FAD assessment method. However, the FAD procedures for critical size analysis, i.e., the largest acceptable crack size, in API 579 is not consistent with crack initiation criteria in references such as, I. Milne, R. A. Ainsworth, A. R. Dowling and A. T. Stewart: “Assessment of integrity of structures containing defects”, CEGB report R/H/R6—Revision 3, 1986; and M. Janssen, J. Zuidema and R. J. H. Wanhill: “Elastic-Plastic fracture mechanics”, Fracture Mechanics, Part III, Chapter 8, pp. 198–203, Delft University Press (2002).
FAD procedures in API 579 for critical size analysis is more appropriate when material failure is expected to be either brittle or is preceded by only a limited amount of ductile tearing. For materials exhibiting significantly ductile tearing prior to failure, prediction of leak or rupture conditions is conservative and does not provide consistent results with field observed “leak-before rupture” incidents. This is because the resulting increase in toughness involved during crack growth is not taken into account in the analysis.
Accordingly, there is a need for a method for detecting a leak before rupture that accounts for the contribution of significant ductile tearing that provides a more accurate prediction by taking into account an of increase in the material fracture toughness involved during crack growth by a ductile tearing mechanism.